1. Field of the Invention
The present invention relates generally to aluminum and alumina mineral-extraction processes, and more specifically to processes which include a basic aluminum alkali sulfate as an intermediate-stage product instead of aluminum hydroxide.
2. Description of the Prior Art
Conventional methods for producing aluminum are not environmentally friendly. The aluminum industry is estimated to produce at least forty million metric tons of greenhouse-gas emissions worldwide each year. The use of so-called xe2x80x9cinert anodesxe2x80x9d in the production of aluminum has long been recognized as a solution to these emissions, but the high electrolysis temperatures needed, e.g., 950xc2x0 C., have proven too challenging for commercial production.
The xe2x80x9cBayer processxe2x80x9d is the most common, and uses a caustic as an extractant on alumina feedstock. Both U.S. Pat. Nos. 5,124,008, and 5,997,828, issued to the present inventor, John Rendall, describe methods for producing alumina from bauxite and clay ore bodies by using sulfuric acid. Both are incorporated herein by reference.
Prior art systems allow both iron and aluminum values to be in solution together within the leach liquor. The caustic in the Bayer process solubilizes some silica, and loses both the alumina and caustic in the form of aluminum silicate.
It is therefore an object of the present invention to provide a process for the efficient extraction of alumina, iron oxide and titanium dioxide from bauxite ore and clays.
It is another object of the present invention to provide an environmentally safer system for the extraction of alumina, iron oxide and titanium dioxide from bauxite ore and clays.
Briefly, a process embodiment of the present invention is a method for the extraction of alumina, iron oxide and titanium dioxide from bauxite ore and clays, and other ore bodies and feedstocks. The process starts with a sulfuric acid leaching of the feedstocks in pressure autoclaves at 180-250xc2x0 C. and appropriate pressure. A leach liquor of sulfate salts of aluminum, iron and titanium is obtained. Any iron values are converted to a ferrous state. A recycled potassium sulfate helps produce double aluminum alkali sulfate crystals in the reduced leach liquor. The crystals are removed at 20xc2x0 C. to 60xc2x0 C. with the help of SO2 gases that reduce the ferric ions to ferrous. Such double salt is hydrolyzed into a basic aluminum alkali precipitated sulfate salt. This is then dried and calcined at about 950xc2x0 C. Any alkali sulfate is washed out and recycled. The remainder is alumina. The ferrous sulfate is crystallized out at 10xc2x0 C. It is dried and calcined at about 450xc2x0 C. to produce an iron oxide.
Such calcination of both alumina and iron sulfates produces gases (SO2+SO3) which are recycled to an acid plant. A bleed stream from a recycled raffinate is used to remove a titanium salt by precipitation at 90xc2x0 C., e.g., using seeding and steam sparging. The bleed stream from the recycled raffinate is removed when the ratio of ferrous sulfate to TiO2 is about 0.8:1. A TiO2 precipitate is removed, washed, and converted for use in commercial products. Another TiO2 removal system raffinate bleed stream helps purge impurities that would otherwise build up. Another way to remove impurities is by evaporation drying and calcining of the metal values.
An advantage of the present invention is that a process is provided that can be used at temperatures significantly lower in electrolysis for production of aluminum and therefore reduces the challenge of inert anode use at this reduced temperature.
Another advantage of the present invention is that a process is provided that significantly improves over conventional processes by converting crystallized double salt aluminum alkali sulfate into a precipitated basic double salt. It recycles about sixty to seventy percent of the water, sulfuric acid and alkali sulfate.
A still further advantage of the present invention is that a process is provided that produces iron oxide and titanium values available as hydroxides.
Another advantage of the present invention is that a process is provided that effectively overcomes conventional problems by using potassium or other alkali sulfate carriers in double salt crystallization and recycling such carriers. The benefits are the coproduction of ferrous sulfate or iron oxide for making steel. The alumina yield also increases, because silica does not affect the reaction.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment as illustrated in the drawing figure.